1. Field of the Invention
The present invention relates to a method for molding an optical fiber fusion spliced portion, and an optical fiber with a molded fusion spliced portion. More particularly, the present invention relates to a method for molding an optical fiber fusion spliced portion to mold (recoat) an exposed portion of the fusion spliced optical fiber again, for example.
2. Description of the Related Art
It is required to produce a long distance optical transmission line in which the closure or terminal box can not be installed, for example, an optical fiber submarine cable of several tens kilometers long. In manufacturing the optical fiber cable, the optical fibers being shorter than a certain cable length are spliced with the characteristics of high strength. In such optical fiber cable, the shape and the outer diameter of the spliced portion must be almost equivalent to those of the coating of the optical fiber.
In related art, the optical fibers are fusion spliced on end faces with the coating removed, and an exposed fusion spliced portion is molded with a ultraviolet curable resin in such a way that it is covered with the ultraviolet curable resin and the resin is cured by applying a ultraviolet ray.
However, in this method for molding the optical fiber fusion spliced portion, the molding resin is applied to be thinner in an overlap portion between the fiber coating and the molding resin so that a diameter of the molded fusion spliced portion is almost equal to that of the coating of the optical fiber. Therefore, there are some cases where a braking occurs from an end of a mold/coating interface between the mold coating and the fiber coating due to a shrinkage of the injected molding resin when cured, and thus a crack occurs on the mold/coating interface.
Thus, a molding method was disclosed in JP-A-5-264848, for example. In this molding method, as shown in FIGS. 6A and 6B, a coating 101a at a top end portion of an optical fiber 100a, 100b to be spliced is partially removed to make the outer diameter of the coating 101 smaller, that is, to form a smaller diameter portion 102. Then, the coating 101a of the top end of the smaller diameter portion 102 is completely removed to fusion splice the optical fibers 100a and 100b on the exposed end faces. A fusion spliced portion 103 and its neighboring portion 104 are molded with a molding resin 105 to have a diameter almost equal to that of the coating 101 of the optical fiber 100a, 100b. 
In the molding method as described above and shown in FIGS. 6A and 6B, the outer diameter of a portion molded with the molding resin 105 can be consistent with the outer diameter of the coating 101 of the optical fibers 100a and 100b, however, the following problems remain.
That is, in an overlap portion 106 where the molding resin 105 and the coating 101 of the optical fibers 100a and 100b overlap, there is a problem on the structure that the thickness of the molding resin to be molded is smaller.
In addition, since the molding resin 105 for use in molding has a large curing shrinkage percentage, and small tensile strength and small tensile elongation, there remains a problem that it is not possible to fully prevent the crack on an interface 107 between the molding resin 105 and the coating 101 that is caused by shrinkage of the injected molding resin 105, when cured. In particular, the crack is more likely to occur when the screening (added tension) recoiling is performed.
Further, the molding resin 105 has a small adhesion with the coating 101 being the outermost layer of the optical fibers 100a and 100b, in addition to the problem that the thickness of the molding resin 105 is small in the overlap portion 106. Therefore, there is also a risk that the overlap portion 106 is exfoliated when making a wiping process with ethanol (ethanol wiping) to remove the excess resin such as fin-like object after curing the molding resin 105.
Moreover, though a sand paper is employed to make the coating 101a at the top end of the optical fiber 100a, 100b to be spliced thin, a quite high level of skills and a lot of time are required, and the working efficiency is worse, because the thickness of the coating 101 is as small as about 60 μm. Further, there is a risk that the sand paper damages the glass, thereby resulting in reducing the fiber strength.